The Vehicle Infrastructure Cooperative System (VICS) may implement vehicle-vehicle information interaction and sharing and vehicle-road information interaction and sharing based on technologies, such as wireless communication, and also may implement intelligent cooperation between the vehicle and infrastructure. Therefore, the target of optimizing and utilizing system resources, enhancing road traffic safety, and alleviating traffic congestion may be achieved.
At present, during the research of the 3rd Generation Partnership Project (3GPP) standard, three synchronization priorities are configured in the Long Term Evolution (LTE) Device to Device (D2D) communication standard. The three synchronization priorities are: in-coverage User Equipment (UE)>partial-in-coverage UE>out-of-coverage UE, which are organized from high to low in order of priority. For instance, as shown in FIG. 1, the standard defines that the synchronization priority may be distinguished by one bit of in coverage flag bit combination, which is transmitted via a Side Link Synchronization Signal (SLSS) synchronization sequence and Physical Side Link Broadcast Channel (PSBCH).
The specific processing includes:
the in-coverage UE: the synchronization sequence is in the id_net set, and is in a range of [0, 167]. The in coverage flag of the synchronization sequence is True;
the partial-in-coverage UE: the partial-in-coverage UE is in synchronization with the in-coverage UE. The synchronization sequence transmitted by the partial-in-coverage UE is the same as the synchronization sequence of a synchronization reference UE (in-coverage UE) selected by the partial-in-coverage UE, however, the in coverage flag of the partial-in-coverage UE is False;
The out-of-coverage UE are divided into three kinds of situations:
1) the selected synchronization reference UE is the partial-in-coverage UE, the number of the transmitted synchronization sequence is the number of the synchronization sequence of the reference UE (in-coverage UE)+168, the synchronization sequence is in the id_oon set, the value of the synchronization sequence is in the range of [168, 335] and the in coverage flag of the synchronization reference UE is False;
2) the selected synchronization reference UE is the out-of-coverage UE, the transmitted synchronization sequence is the same as the synchronization sequence of the selected reference UE (out-of-coverage UE), the synchronization sequence is in the id_oon set, the value of the synchronization sequence is in the range of [168, 335] and the in coverage flag of the synchronization reference UE is False;
3) this kind of UE does not select any synchronization reference UE and forms an independent synchronization source, the synchronization sequence is in the id_oon set, the UE randomly selects a sequence from [168, 335], configures the selected sequence as its synchronization sequence and transmits the synchronization sequence, the in coverage flag of this kind of UE is False.
There may be two kinds of synchronization sources in the Internet of vehicle communication. Such as, the synchronization source may be a node of the Global Navigation Satellite System (GNSS) or may be the eNB.
In the LTE D2D communication, in the Centralized synchronous network, the central node of which is the eNB, the in-coverage UE may forward the synchronization obtained from the eNB, so that the UE, which is out of the coverage of the eNB, may obtain the synchronization of the in-coverage UE. In the scenario of the partial coverage, the UE, which is at the edge of the network coverage, also can receive the D2D Synchronization Signal (D2DSS) from the in-coverage UE, the out-of-coverage UE may receive the D2D signal from the in-coverage UE based on the D2DSS. As for the out-of-coverage UE, which is far from the eNB, an ISS-based synchronization network may be formed. In the LTE D2D, the node, which directly obtains the GNSS, is not taken as the synchronization source. The synchronization scheme only takes the eNB as the core and sends out the clock of the eNB.
In the Internet of vehicle communication, compared with the mobile phone, the vehicle node has more processing power. The vehicle node may directly obtain reliable synchronization signals, may be directly configured as the synchronization source, may expedite the synchronization convergence, improve synchronization accuracy, form a uniform timing and reduce number of synchronization clusters.
Therefore, the design of the synchronization information transmission scheme of the Vehicle Network should be modified to support the synchronization communication in the Vehicle Network. In the VICS, compared with the synchronization system, which is formed taking the eNB as the center by the LTE D2D, due to the introduction of the GNSS, multiple synchronization sources may provide the synchronization information, the synchronization convergence may be expedited and the number of the synchronization clusters may be reduced. However, till then, a reasonable synchronization information transmission method, with which the UE may determine the synchronization priority and perform synchronization adjustment, has not been formed.